1. Field of the Invention
The present invention relates to an audio signal amplifier circuit and electronic apparatus having the same and more specifically an audio signal amplifier circuit which is used in an audio system for a portable type electronic apparatus such as a cellular phone, an immobile phone and a portable terminal device and an electronic apparatus such as a personal computer and which permits to reduce signal distortion even when driven under a comparatively low voltage and is suitable for forming in an integrated circuit having a broad dynamic range.
2. Conventional Art
Conventionally, many audio systems for a portable type electronic apparatus such as a cellular phone, an immobile phone and a portable terminal device and an electronic apparatus such as a personal computer are operated under a relatively low voltage such as about DC 6V of source voltage or less than such voltage, which caused a problem of narrowing the dynamic range thereof.
As a low voltage drive amplifier circuit which performs a push-pull operation suitable for a portable type electronic apparatus such as a cellular phone and a portable terminal device, a circuit wich uses a current mirror circuit at a drive stage and enlarges the dynamic range is known from JP-A-5-308228 or JP-A-9-46146.
In an acoustic use audio system, a power amplifier is used of which distortion rate is suppressed low and dynamic range is broadened through the use of FET transistors in a MOS circuit at the final stage, which is known from JP-A-11-103216. Further, as an operational amplifier of the same kind, a Bi-CMOS circuit in which an output stage in a C-MOSFET at the final stage is driven by a bipolar transistor is well known.
In an audio system for a portable type electronic apparatus such as a cellular phone, an immobile phone and a portable terminal device and an electronic apparatus such as a personal computer, an improvement of the sound quality and an increase of the output thereof are expected, and further such demand is keen. Moreover, a reduction of power consumption is also demanded.
When a bipolar transistor is used at a final stage, if an idling current is not suppressed, there arises a problem of increasing a power loss during no signals. A technology disclosed in JP-A-9-46146 as mentioned previously resolves the above problem. However, the technology has a drawback that the circuit structure of the drive stage somewhat complexes.
Therefore, it is conceived to suppress the idling current through a use of FET transistors in a C-MOS circuit at the final stage, however, when a CMOS operational amplifier in a push-pull operation is used in which a MOSFET is driven by a bipolar transistor under a comparatively low voltage drive less than DC 6V, problems remain unsolved that the drive of the MOSFET can not be performed sufficiently and the dynamic range thereof reduces.
Further, a CMOS operational amplifier in a push-pull operation is used, respective biases at the positive phase side and the opposite phase side have to be set differently. After thus set, the final stage has to be driven, therefore, number of poles (bend points) on Bode diagram (graph of frequency vs. total gain) increases, if driven under a low voltage, cross over distortion increases, and if the output is increases an oscillation likely occurs.